Preproapelin is a 77 amino acid protein expressed in the human CNS and peripheral tissues, e.g. lung, heart, and mammary gland. Peptides comprising C-terminal fragments of varying size of apelin peptide were shown to activate the G protein-coupled receptor, APJ receptor (now known as APLNR) (Habata, et al., 1999, Biochem Biophys Acta 1452:25-35; Hosoya, et al., 2000, JBC, 275(28):21061-67; Lee, et al., 2000, J Neurochem 74:34-41; Medhurst, et al., 2003, J Neurochem 84:1162-1172). Many studies indicate that apelin peptides and analogues convey cardiovascular and angiogenic actions through their interaction with the APJ receptor (APLNR), such as endothelium-dependent vasodilation (Tatemoto et al., 2001, Regul Pept 99:87-92.
APLNR is a Class A (rhodopsin-like) G protein-coupled receptor (GPCR). Although monoclonal antibodies offer an alternative approach to small molecule compounds or peptidomimetics for modulating APLNR, a number of factors make it particularly difficult to generate therapeutic antibodies that bind to and modulate GPCRs. For one, target epitopes (ligand binding domains) of a GPCR are often embedded in the plasma membrane. Furthermore, generating an immune response to GPCR proteins in host animals can be problematic since mammals share high structural homology for these membrane proteins and therefore may not recognize the GPCR antigen as foreign (Herr, D R, 2012 Intl Rev Cell Mol Biol 297:45-81). Still, antibodies to GPCRs remain promising for therapeutic intervention if the antibody can instill an active or nonactive conformation (e.g. agonism or antagonism) of the receptor or prevent (neutralize) binding of the receptor's endogenous ligand.
The apelin system appears to play a role in pathophysiological angiogenesis. Studies have indicated that apelin may be involved in hypoxia-induced retinal angiogenesis (Kasai et al., 2010, Arterioscler Thromb Vasc Bioi 30:2182-2187). In some reports, certain compositions may inhibit angiogenesis by inhibiting the apelin/APJ pathway (see, e.g., U.S. Pat. No. 7,736,646), such as APLNR inhibitors capable of blocking pathological angiogenesis and therefore useful in inhibiting tumor growth or vascularization in the retina (Kojima, Y. and Quertermous, T., 2008, Arterioscler Thromb Vasc Biol; 28; 1687-1688; Rayalam, S. et al. 2011, Recent Pat Anticancer Drug Discov 6(3):367-72). As such, interference with apelin-mediated signaling may also be beneficial in early prevention of proliferative diabetic retinopathy (Tao et al., 2010, Invest Opthamol Visual Science 51:4237-4242; Lu, Q. et al, 2013, PLoS One 8(7):e69703).
Apelin has also been reported in the regulation of insulin and mechanisms of diabetes and obesity-related disorders. In mouse models of obesity, apelin is released from adipocytes and is directly upregulated by insulin (Boucher, et al., 2005, Endocrinol 146:1764-71). Apelin knockout mice demonstrate diminished insulin sensitivity (Yue, et al., 2010, Am J Physiol Endocrinol Metab 298:E59-E67).
Furthermore, APLNR agonists may mimick apelin-induced vasodilation and angiogenesis to be protective in ischemia-reperfusion injury and improve cardiac function in conditions such as congestive heart failure, myocardial infarction, and cardiomyopathy. Therapeutic administration of apelin peptides reportedly contributes to the promotion of angiogenesis and functional recovery from ischemia. (Eyries M, et al., 2008, Circ Res 103:432-440; Kidoya H, et al., 2010, Blood 115:3166-3174). The small molecule E339-3D6, which behaves as a partial agonist with respect to cAMP production, induces vasorelaxation in an ex vivo precontracted rat aorta model (Iturrioz, X, et al., 2010 FASEB 24(5):1506-1517).
APLNR signaling, and modulation thereof, has been implicated as a factor in a variety of diseases and disorders (e.g. WO2004081198A2, published on 23 Sep. 2004), and there is still a need for therapeutic agents that modulate APLNR biological activity.